A Key to Key Signatures

A Sharp:
A Flat:
A Natural:
A Double Sharp: A Double Flat:

raises a note by one semitone.
lowers a note by one semitone.
cancels out a previous accidental, thus returning the note to its original pitch raises a note by two semitones, or a whole note. lowers a note by two semitones, or a whole note.

Key Signature: Whenever there is a group of one or more sharps or flats right after the clef sign, all the notes of those letter names throughout the whole piece are affected accordingly. The sharp or flat note can be cancelled, though, if there is another sharp or flat within the piece.